Mexico’s vast reserves of oil and natural gas will be open to exploration by foreign companies beginning in 2015, thanks to a historic change proposed by President Enrique Peña Nieto and approved by Mexico’s Congress on August 6.

The change, which required a revision to Mexico’s constitution, opens the way for global oil companies to access untapped oil reserves that state-owned Pemex (Petroleos Mexicanos) has estimated at 113 billion barrels worth $11 trillion.

In addition, Pemex estimates that five shale fields it has identified hold 460 trillion cubic feet of natural gas, worth $2.2 trillion. One of those fields is just across the border from the richly productive Eagle Ford field in Texas.

Pemex has had a monopoly on the country’s energy sector since Mexico nationalized the industry in 1938.

While oil and gas production has soared in the United States and most other oil-producing nations, output by Pemex has declined for nine consecutive years, recently reaching its lowest level since 1995. Pemex has only four deepwater oil platforms in the Gulf of Mexico, compared to 53 in U.S. waters.

Proponents of the move to open Mexico’s fields to foreign producers say they expect production to increase by 20%, to more than 3 million barrels per day, by 2018.

The reform also ends the monopoly on electricity generation and distribution in Mexico.